In the field of intelligent communications controllers, many controllers contain intelligent logic modules, such as microprocessors. These microprocessors are designed with clocking output signals that can be readily used by custom communications electronic devices that are used to transfer data for the microprocessor. The Z80SIO is a communications device that transfers data to various communications ports from the communications module wherein the microprocessor and Z80SIO (serial input-output) device is contained. Typically, the microprocessor transfers data between itself and the communications device. The communications device transfers that data over a communications line to a plurality of communications ports. The benefit of this type of transfer is that it decreases the use of the processing power of the microprocessor on data communications.
The sequential operation of these communications chips is synchronized with the microprocessor. The microprocessor communicates various clocking signals to the communications device during the execution of various microinstructions. These microinstructions are executed within variable instruction cycles. The various clocking signals synchronize the execution rate of the SIO device and the execution rate of the microprocessor. Thus, any variation in the microprocessor instruction cycle has a timing transfer effect on the communications device. Consequently, the communications device has a variable transfer rate. Moreover, when the microprocessor is executing instructions which have a minimum instruction cycle time, the SIO device is transferring at a maximum rate. When the microprocessor executes instructions which are longer than the minimum instruction cycle time, the communications device experiences a slower transfer rate.
Thus, one of the major disadvantages of modern communications controllers using the communications device is that they will experience a variable transfer rate. It is beneficial for computer systems not to experience variable transfer rates, but beneficial to maintain the highest transfer rate possible at all times.